This invention relates to the field of petrochemical synthesis, and in particular, to a method of production of phenol, acetone and alpha-methystryrene (AMS) by the cumene method.
There are a number of methods known to produce phenol and acetone by using acidic cleavage of technical cumene hydroperoxide (CHP). The main difference between the known methods is in using different reaction mediums and alternate techniques for removing the heat (380 Kkal/kg) generated during the process of CHP cleavage.
In these prior art processes the best selectivity is obtained by using an equimolar mixture of phenol and acetone as a reaction medium. On a relative basis, 15-30% of acetone, based on technical CHP, is added to this mixture. This is illustrated in Russian Application No. 9400736/04/007229 dated Mar. 1, 1994 and U.S. Pat. No. 4,358,618. This allows one to obtain a good process selectivity as determined by the yield of the desired by-product, AMS, formed from dimethylbenzene alcohol (DMBA) present in technical cumene hydroperoxide. The obtained AMS yield in is about 80%.
During the CHP cleavage, the heat generated is removed. In the process according to U.S. Pat. No. 2,663,735 the heat is removed by acetone evaporation and acetone recycle to the reactor. The generated heat can also be removed by the use of a cooling medium such as cooling water.
During the adiabatic cleavage of 100% CHP the temperature is increased to about 700.degree. C. under the influence of an acidic catalyst. The heat is generated spontaneously. Because of the rapid heat release, the CHP cleavage process is considered very dangerous. Consequently, the combination of heat generation and heat removal is of high priority for improving process safety.
In the process of U.S. Pat. No. 2,663,735, the reaction heat is removed by acetone evaporation and the heat generation and the heat removal are fully combined. The heat generated in the process of cleaving 1 ton of CHP requires feeding approximately 2.2-3 tons of acetone to the reactor. The evaporated acetone is to exhausted from the reactor, condensed and continuously recycled to the reactor. As a result, the reactor is operated in a heat stable manner as is required for process safety.
However, the heat stable condition is obtained only by the use of a comparatively high sulfuric acid concentration of 1200-1300 ppm. However, the high H.sub.2 SO.sub.4 concentration, which is needed due to the large amount of acetone fed into cleavage products, decreases the activity of sulfuric acid which is the CHP cleavage catalyst. Thus, high acid concentration results in a low yield of desired products and a high content of microimpurities (about 1500 ppm) such as mesithyl oxide, hydroxyacetone, and 2-methylbenzofurane which substantially adulterate the phenol quality. While the process chemistry requires a low sulfuric acid concentration of about 100-300 ppm, this can not be achieved in practice since CHP accumulates in the reactor bottoms because of the sharp decrease of the CHP cleavage rate that results in a large heat release. i.e. when decreasing the sulfuric acid concentration the reactor is operated under unstable heat conditions. Actually the process achieves thermal stability only at a high sulfuric acid concentration but this results in a low process selectivity. Therefore, in the process employing acetone evaporation, the objectives of heat stability and obtaining a high process selectivity are in irreconcilable conflict.
In the processes of the above referenced Russian application, U.S. Pat. Nos. 4,358,618 and 5,254,751, reaction heat is removed with the reaction products or reaction cleavage mass (RCM) by multiple circulations through water cooled heat exchangers. The heat exchangers, which may number from 2 to 6, are in fact the reactors wherein the CHP cleavage occurs. The heat stability of the process (i.e. the process safety) depends on the composition of the reaction products, the range of the acid concentration, the temperature profile and, hence, CHP conversion distribution in the reactors. The process stability deteriorates at higher CHP conversion in the first reactor and as the temperature difference between the first and the subsequent reactors increases. In practice, the more the conditions of the process are non-isothermal, the more precarious is the process state.
In the process according to the Russian application, the CHP and DCP cleavages are performed in two stages. CHP cleavage reactors (mixing reactors) and the DCP conversion reactor (plug-flow reactor) are operated at the same pressure.
The CHP and DCP cleavage are performed in an equimolar mixture of phenol and acetone containing up to 12 wt % of cumene. To reduce the acidic properties of the sulfuric acid and, therefore, to increase the yield of such desired products as phenol, acetone and AMS, additional acetone is added into the reaction products according to the following algorithm: EQU G.sub.ac. =G.sub.CHP .times.0.125 [CHP]+35/(G.sub.CHP .times.[CHP]),
where:
G.sub.ac., G.sub.CHP represent the flow rate of additional acetone and technical CHP, respectively, in kg/hr and PA1 [CHP] is CHP concentration of technical grade CHP (wt %) PA1 1. Cumene (isopropylbenzene) oxidation with air and/or oxygen to cumene hydroperoxide (CHP); PA1 2. Acidic (H.sub.2 SO.sub.4) cleavage of the produced CHP; and PA1 3. Rectification of CHP cleavage products by the method of multi-step rectification
that is equal to 12-14% rel. of acetone based on technical CHP feed rate.
CHP conversion, depending on the feed rate, is maintained in the first reactor at 62-75%, in the second reactor at 87-94% and in the third reactor at 94-98%. The corresponding temperatures in these reactors are 67-79.degree. C., 78-67.degree. C. and 69-60.degree. C., respectively. The above algorithm for the feeding of additional acetone, the temperature, and CHP conversion distribution in the reactors allow the process to operate within a wide range of feed rates.
The CHP concentration at the outlet of the reactors of the first stage is 0.14-0.43 wt.-% that corresponds to a .DELTA.T of 1-3.degree. C. in the calorimeter which controls the first stage of the process
Water is fed to the DCP cleavage reactor in an amount so as to provide a water concentration in the reaction products of 1.3-2.0 wt.-%. The operation of the reactor of the second stage is controlled by .DELTA.T equal to 1-3.degree. C. of the calorimeter installed in the line before the DCP cleavage reactor. In the DCP cleavage reactor the process conditions are isothermal. Different temperatures from 94.degree. C. at the low feed rates to 99.degree. C. at high feed rates are maintained in the DCP cleavage reactor. The entire process (1st and 2nd stages) is controlled by the temperature differential between the two calorimeters. This calorimeter temperature differential .DELTA. is 0.2-0.3.degree. C.
In order to reduce non-selective losses in the acetone flash stage, ammonia is added into the line before the evaporator to convert sulfuric acid into the neutral salt (NH.sub.4).sub.2 SO.sub.4. As a result, AMS yields of 78.8-79.6% of theory are obtained in the process.